System and method for LLC converter design

Abstract

An embodiment method for designing a power converter system includes receiving, by a processor, power converter design parameters. The design parameters include a minimum DC input voltage Vmin and a maximum DC input voltage Vmax, a minimum switching frequency fmin and a maximum switching frequency fmax of a switching bridge of the power converter, and a target output voltage and a target output power. The method also includes calculating, by the processor, a first power converter configuration. The first power converter configuration includes a calculated magnetizing inductance Lmc equal to Re tan(φ)(2πfmin)−1, where φ is a load angle complement equal to a sin(VminVmax−1), and Re is an equivalent reflected load resistance of the power converter. The first power converter configuration also includes a calculated resonant inductance Lrc equal to Lmc cos2(φ)(fmax2fmin−2−1)−1 and a calculated resonant capacitance Crc equal to Lrc−1(2πfmax)−2.

Description

TECHNICAL FIELD

The present invention relates generally to a system and method for designing Direct Current-to-Direct Current (DC-to-DC) power converters, and, in particular embodiments, to a system and method for inductance-inductance-capacitance (LLC) converter design.

BACKGROUND

DC-to-DC power converters are desired for many applications such as computer server systems and portable consumer electronics. Some DC-to-DC converters employ frequency switching that increases voltage gain to compensate for a partially lowered input voltage and thereby provide increased reliability, higher power density, and improved output voltage regulation. Additionally, LLC converters may support zero voltage switching to reduce switching losses and increase efficiency. LLC converters that are designed to operate within a specific range of switching frequencies may be used to reduce interference from electromagnetic signals and to switching losses and component size.

Nevertheless, designing such LLC power converters presents a number of challenges. Existing converter design techniques focus on achieving a particular voltage gain, but neglect design parameters for switching frequency range. Furthermore, these existing techniques are not capable of being automated. Additionally, existing electronic design automation (EDA) software tools allow designers of electronic systems such as printed circuit boards and integrated circuits to design and analyze entire semiconductor chips in a design flow. Yet these EDA tools do not currently support designing LLC power converters from given input and output design parameters.

SUMMARY

In accordance with an embodiment of the present invention, a method for designing a power converter system is provided. The method includes receiving, by a processor, power converter design parameters. The design parameters include a minimum DC input voltage V_min and a maximum DC input voltage V_max, a minimum switching frequency f_min and a maximum switching frequency f_max of a switching bridge of the power converter, and a target output voltage and a target output power. The method also includes calculating, by the processor, a first power converter configuration. The first power converter configuration includes a calculated magnetizing inductance L_mc equal to R_etan(φ)(2πf_min )⁻¹, where φ is a load angle complement equal to asin(V_minV_max⁻¹), and R_e is an equivalent reflected load resistance of the power converter. The first power converter configuration also includes a calculated resonant inductance L_rc equal to L_mccos²(φ)(f_max²f_min⁻²−1)⁻¹ and a calculated resonant capacitance C_rc equal to L_rc⁻¹(2πf_max)^−2.

In accordance with another embodiment of the present invention, a power converter design system is provided. The system includes a non-transitory computer-readable medium storing programming. The programming includes instructions to receive power converter design parameters. These design parameters include a minimum DC input voltage V_min and a maximum DC input voltage V_max, a minimum switching frequency f_min and a maximum switching frequency f_max of a switching bridge of the power converter; and a target output voltage V_o and a target output power P_o. The programming also includes instructions to calculate a first power converter configuration, which includes a calculated magnetizing inductance L_mc equal to R_e tan(φ)(2πf_min)⁻¹, where φ is a load angle complement equal to a sin(V_minV_max⁻¹), and R_e is an equivalent reflected load resistance of the power converter. The first power converter configuration also includes a calculated resonant inductance L_rc equal to L_mc cos²(φ)(f_max²f_min⁻²−1)⁻¹ and a calculated resonant capacitance C_rc equal to L_rc⁻¹(2πf_max)⁻².

In accordance with another embodiment of the present invention, a power conversion system is provided. The system includes a switching bridge that includes a plurality of switches coupled to a DC power source having a minimum input voltage V_min and a maximum input voltage V_max. The switching bridge is configured to switch at a frequency that is not less than a minimum frequency f_min and that is not greater than a maximum frequency f_max. The system also includes a primary side circuit coupled to the switching bridge. The primary side circuit includes a primary winding of a transformer. The system also includes a secondary winding magnetically coupled to the primary winding through a core of the transformer, and an output terminal coupled to the secondary winding. The output terminal is configured to supply an output voltage that is not greater than a maximum output voltage V_o and an output power that is not greater than a maximum output power P_o. The transformer has a magnetizing inductance L_m that is greater than c₁R_e(2πf_min)⁻¹tan(φ) and less than c₂R_e(2πf_min)⁻¹tan(φ), where c₁ is not less than 0.75 and c₂ is not greater than 1.25, where φ is a load angle complement equal to a sin(V_minV_max⁻¹), and where R_e is an equivalent reflected load resistance. The primary side circuit has a resonant inductance L_r that is greater than c₁L_m(f_max²f_min⁻²−1)⁻¹cos²(φ) and less than c₂L_m(f_max²f_min⁻²−1)⁻¹cos²(φ). The primary side circuit has a resonant capacitance C_r in series with the resonant inductance such that C_r is greater than c₁L_r⁻¹(2πf_max)⁻² and less than c₂L_r⁻¹(2πf_max)⁻².

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which

FIG. 1, which includes FIGS. 1A and 1B, is a block diagram showing a DC-to-DC power converter designed in accordance with embodiments of the present invention;

FIG. 2 is a graph illustrating both the converter gain and angle of the converter's input impedance as a function of the converter's operating frequency in accordance with embodiments of the present invention;

FIG. 3 is a vector diagram illustrating expressions for the converter's calculated load impedance and calculated gain when the converter's input impedance angle is zero and when using calculated component values in accordance with embodiments of the present invention;

FIG. 4 is a flow diagram showing a method for designing an LLC converter in accordance with embodiments of the present invention; and

FIG. 5 is a block diagram of a processing system that may be used for implementing some of the devices and methods disclosed herein in accordance with embodiments of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferred embodiments in a specific context, a system and method for LLC converter design for use in EDA and other automated design systems. Further embodiments may be applied to other switched LLC converter design systems that require specifying a range of switching frequencies.

FIG. 1 shows a DC-to-DC power converter designed in accordance with embodiments of the present invention. FIG. 1A is an LLC converter circuit with a DC input coupled to a switching bridge and a transformer. FIG. 1B is an equivalent AC circuit that is a first harmonic approximation of the circuit of FIG. 1A.

FIG. 1A shows a DC-to-DC power converter that is an LLC converter having a parallel inductance, series resonant inductance, and series resonant capacitance on the primary side of a transformer. The power converter includes a switching bridge 102 that has multiple switches coupled to a DC power source providing an input voltage V_in. The DC power source has a minimum input voltage V_min and a maximum input voltage V_max. The switching bridge 102 is configured to provide a switched voltage signal V_sw, which is a square wave that the switching bridge generates by switching at a frequency f_sw that is not less than a minimum frequency f_min and that is not greater than a maximum frequency f_max. The switching bridge may include, for example, Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) switches.

The minimum switching frequency f_min is chosen to be high enough for example, at least 30 to 40 kilohertz (kHz) or higher, to reduce interference from audio signals. The maximum switching frequency f_max is chosen to be approximately equal to a resonant switching frequency f_r of the resonant tank inductor 124 and the resonant tank capacitor 126. The inductance and capacitance of these components are in turn chosen so that the resonant switching frequency f_r is both low enough to reduce switching losses and high enough to reduce the required size of the resonant tank inductor 124 and the resonant tank capacitor 126. In an embodiment, component values are selected to provide a resonant switching frequency/maximum switching frequency in the range of 80 to 200 kHz.

The combined required size of the resonant tank inductor 124 and the resonant tank capacitor 126 is proportional to the combined average peak energy E(f_xo) that is contained in each of these components at the minimum switching frequency, where f_xo is a normalized minimum switching frequency equal to f_min/f_r. In turn, the average peak energy E(f_xo) is proportional to a function ƒ(f_xo), which has a derivative dƒ(f_xo)/df_xo, such that:
ƒ(f_xo)=(1+f_xo²)[(f_xo)(1+f_xo²)]⁻¹   (Eq. 1A)
dƒ(f_xo)/df_xo=f_xo⁴+4f_xo²−1   (Eq. 1B)

E(f_xo) reaches a minimum energy level, and the combined required size of the resonant components is minimized, when the derivative dƒ(f_xo)/df_xo is equal to zero, which occurs when f_xo=0.485. In an embodiment, f_min and f_r are selected so that the required size of the resonant components is not very sensitive to changes in f_xo. For example, if f_xo is limited to vary within a range of (0.34, 0.63), which is centered on 0.485, ƒ(f_xo) varies by around only ten percent (from 3.71 to 3.68), and therefore the required size of the resonant components varies by around only ten percent.

Referring again to FIG. 1A, the power converter also includes a primary side circuit 104 coupled to the switching bridge 102, and this primary side circuit 104 includes a primary winding 106 of a transformer 108. The transformer 108 also includes a secondary winding 110 magnetically coupled to the primary winding 106 through the transformer core 112. The secondary winding 110 is part of a secondary side circuit 114 that includes a rectifier 116 and an output capacitor 118 having a capacitance C_o. The secondary side circuit 114 is coupled at an output terminal to a load 120 having an output resistance R_o. The secondary side circuit supplies the load with an output voltage that is not greater than a maximum output voltage V_o and an output power that is not greater than a maximum output power P_o. The transformer 108 has a magnetizing inductance L_m across the primary winding 106. The primary side circuit 104 has a resonant tank inductor 124 having a resonant inductance L_r, and a resonant tank capacitor 126 in series with the resonant tank inductor 124. The resonant tank capacitor 126 has a resonant capacitance C_r. In other embodiments, part or all of the resonant inductance L_r may be provided by a primary-side inductance of the transformer 108 or other primary-side components, so that a reduced resonant tank inductor or no resonant tank inductor is present. Similarly, in other embodiments part or all of the resonant capacitance C_r may be provided by a primary-side capacitance of the transformer 108 or other primary-side components so that a reduced resonant tank capacitor or no resonant tank capacitor is present.

The primary winding 106 has a number of turns n times greater than or less than a number of turns y of the secondary winding 110. In embodiments of the present invention, the transformer is selected so that this turns ratio n is within plus or minus 25% of a calculated turns ratio n_c. In an embodiment, the calculated turns ratio n_c is in a range from [99%×V_max(sV_o)⁻¹]≦n_c≦[V_max(sV_o)⁻¹], where s is a switching factor of the switching bridge. This range of allowable values for n_c takes into account voltage losses that may occur.

In a first embodiment, the power converter is a half-bridge converter in which the switching bridge 102 includes two switches and the switching factor s is equal to 2. In a second embodiment, the power converter is a full-bridge converter in which the switching bridge 102 includes four switches and the switching factor s is equal to 1. In embodiments of the present invention, the rectifier 116 may also be either a half-bridge rectifier made up of two diodes or a full-bridge rectifier made up of four diodes. In other embodiments, the rectifier is a synchronous rectifier.

FIG. 1B shows an equivalent AC circuit based on a first harmonic approximation of the circuit of FIG. 1A. The equivalent AC circuit replaces the primary winding, the transformer core, and the secondary side circuit with a reflected load resistance 128 having a resistance R_e equal to 8π⁻²n_c²V_o²P_o⁻¹. The equivalent AC circuit also replaces the DC voltage source and the switching bridge with an AC voltage source 122 that provides a sinusoidal input voltage V_{in_ac} at the frequency f_sw of the switched voltage signal V_sw. The equivalent AC circuit operates at an angular operating frequency w corresponding to the switching frequency such that ω=2πf_sw.

The equivalent AC circuit has a load impedance that is the product over the sum of the impedances of the reflected load resistance R_e and the magnetizing inductance L_m such that:
Z₁(ω)=jωL_mR_e(R_e+jωL_m)⁻¹=[ωL_m+jR_e][ω_oL_mR_e⁻¹+R_e(ω_oL_m)⁻¹]⁻¹   (Eq. 2)

The equivalent AC circuit also has a resonant impedance Z_r(ω) that is the sum of the impedances of the resonant capacitance C_r and the resonant inductance L_r:
Z_r(ω)=jωL_r+(jωC_r)⁻¹=jωL_r(1−L_r⁻¹C_r⁻¹ω⁻²)   (Eq. 3)

By substituting an angular resonant frequency ω_r that is equal to (L_rC_r)^−0.5, Equation 3 may be rewritten as:
Z_r(ω)=−jωL_r[(ω_r/ω)²−1]  (Eq. 4)

This angular resonant frequency ω_r corresponds to a resonant switching frequency f_r that is equal to 2π(L_rC_r)^−0.5. When the angular operating frequency w is equal to this angular resonant frequency ω_r, a resonance occurs such that the resonant impedance Z_r is equal to zero and maximum current flows through the series resonant capacitance C_r and series resonant inductance L_r.

The equivalent AC circuit also has an input impedance that is equal to the sum of the resonant impedance and the load impedance such that Z_i(ω)=Z₁(ω)+Z_r(ω). Because the real component of the input impedance is always positive, the sign of the angle of the input impedance is the same as the sign of the imaginary component of the input impedance Im(Z_i), such that when the angle of the input impedance is less than 0 the LLC converter operates in capacitive mode and the input voltage lags behind the input current. The MOSFET body diodes in the switching bridge will then be exposed to hard commutation which dramatically increases switching losses.

To avoid these switching losses and to allow for MOSFET soft-switching during startup, the LLC converter is instead operated in inductive mode such that input current lags behind the input voltage. This LLC operates in this inductive mode at frequencies that are high enough that the angle of the input impedance and accordingly the imaginary component of the input impedance are greater than or equal to zero.

The equivalent AC circuit also has a transfer function having a gain G(ω) equal to V_{o_ac}V_{in_ac}⁻¹. As will be explained in connection with FIG. 2, a maximum gain is achieved when the LLC converter is operated at a lowest angular operating frequency ω_o that is still above the capacitive operating range.

Referring to both FIGS. 1A and 1B, the components of the LLC converter are chosen to satisfy design parameters that include: the minimum DC input voltage V_min and the maximum DC input voltage V_max, the minimum switching frequency f_min and the maximum switching frequency f_max of the switching bridge 102, a target output voltage V_o, and a target output power P_o. The selected components support a zero-angle operating frequency ω_o that corresponds to the minimum switching frequency f_min parameter such that ω_o≈2πf_min, so that the converter always operates in inductive mode. The selected components support an angular resonant frequency ω_r that corresponds to the maximum switching frequency f_max parameter such that ω_r≈2πf_max, so that the converter always operates in boost mode below the resonant frequency.

The selected components of the LLC converter also support a maximum gain G_max corresponding to the minimum and maximum input voltage parameters such that G_max≈V_maxV_min⁻¹, so that the gain may be increased from a minimum gain when V_in is equal to V_max up to a gain of V_maxV_min⁻¹ when V_in is equal to V_min. The gain is increased by decreasing the switching frequency of the switching bridge 102 when the input voltage drops. The selected components also support an equivalent reflected load resistance R_e, which as described earlier is a function of the calculated turns ratio n_c. Thus, R_e corresponds to the target output voltage and maximum input power such that [8(πs)⁻²(99%×V_max)²P_o⁻¹]≦R_e≦[8(πs)⁻²V_max²P_o⁻¹].

In particular, the LLC converter components are selected so that actual values of the turns ratio n, magnetizing inductance L_m, resonant capacitance C_r, and resonant inductance L_r are each within plus or minus 25% of respective calculated values n_c, L_mc, C_rc, and L_rc, which are calculated based on the power converter's design parameters. An expression for the calculated turns ratio n_c is previously described. The calculated magnetizing inductance L_mc is equal to R_e(2πf_min)⁻¹tan(φ), where φ is a load angle complement equal to a sin(V_minV_max⁻¹). The calculated resonant inductance L_rc is equal to L_mc(f_max²f_min⁻²−1)⁻¹cos²(φ). The calculated resonant capacitance C_rc is equal to L_{r_c}⁻¹(2πf_max)⁻². FIGS. 2 and 3 illustrate how the foregoing expressions for calculating L_mc, C_rc, and L_rc component values support design parameters such that ω_o≈2πf_min, ω_r≈2πf_max, and G_max≈V_maxV_min⁻¹. Additionally, examples of using calculated values for two LLC converter designs are shown in Table 1 below, along with actual component values of two demonstration boards that were designed in accordance with the calculated values:

TABLE 1
Calculated Vs. Actual Component Values
Bridge	Vmin	Vmax	Vo	Po	fmin	fmax		Lr	Cr	Lm
Type	(V)	(V)	(V)	(W)	(kHz)	(kHz)		(μH)	(nF)	(μH)	n
Half	350	384	12	600	90	157	Calc.	15.9	64.6	192	15.9
(s = 2)							Actual	15.5	66	195	16
							Actual/	97.5%	102.2%	101.6%	110.6%
							Calc.
Full	16	33	400	125	50	110	Calc.	2.48	843	12.5	12.1−1
(s = 1)							Actual	2.3	940	12.2	12−1
							Actual/	92.7%	111.5%	97.6%	100.8%
							Calc.

Referring now to FIG. 2, typical curves are illustrated for both the converter gain G(ω) and input impedance angle. The independent variable ω/ω_r is an angular operating frequency that has been normalized relative to the angular resonant frequency. The gain G(ω) is equal to V_{o_ac}V_{in_ac}⁻¹, which means that G(ω) can be expressed in terms of the load impedance Z₁, the resonant impedance Z_r and the input impedance Z_i as:
G(ω)=Z₁(ω)Z_i(ω)⁻¹=Z₁(ω)[Z₁(ω)+Z_r(ω)]⁻¹   (Eq. 5)

Using Equations 2 and 3, Equation 5 may be inverted to obtain G(ω)⁻¹, the reciprocal of the gain, in terms of the resonant inductance L_r and the angular resonant frequency ω_r:
G(ω)⁻¹=1+Z_r(ω)/Z₁(ω)=1−jωL_r[(ω_r/ω)²−1]Z₁(ω)⁻¹   (Eq. 6)

In these calculations, when the angular operating frequency ω is less than or equal to the angular resonant frequency ω_r, the term [(ω_r/ω)²−1] is greater than or equal to zero so that the converter operates with a gain greater than or equal to one. As the angular operating frequency drops further below the angular resonant frequency, the converter operates in boost mode such that the term [(ω_r/ω)²−1] increases and the gain of the converter increases. Maximum gain is thus achieved at a lowest angular operating frequency that is still above the capacitive operating range (i.e., Im(Z_i)≧0). This condition is fulfilled at the zero-angle operating frequency ω_o where the angle and the imaginary component of the input impedance are equal to zero.

At this zero-angle operating frequency, the magnitude of the gain |G(ω_o)| is maximized and the converter operates in resistive mode with the input current in phase with the input voltage. Using the properties of the gain G(ω_o) at this frequency ω_o, an expression for γ_o, the gain angle at ω_o, can be derived. First, the input impedance of the converter can be written as the product of the load impedance and the reciprocal of the gain such that Z₁(ω)G(ω)⁻¹=|Z_i|exp[j(λ−γ)], where λ−γ is the angle of the input impedance in terms of the angle λ of the load impedance and the angle γ of the gain. When the angle of the input impedance is equal to zero, the difference between the load impedance angle and the gain angle λ_o−γ_o is also equal to zero. Thus, the gain angle γ_o=λ_o, where λ_o is the load impedance angle when the angular operating frequency ω equals ω_o.

Referring now to FIG. 3, a vector diagram is shown that illustrates expressions for a calculated load impedance Z_lc(ω_o) and the calculated gain G_c(ω_o) when the angular operating frequency ω equals ω_o and when calculated component values are used. The real components of the vectors are plotted against the horizontal axis and the imaginary components are plotted against the vertical axis. A change of variables employs a load angle complement φ in terms of γ_oc (the angle of the calculated gain at ω_o) and λ_oc (the angle of the calculated load impedance at ω_o) such that:
φ=−λ_oc+π/2=−γ_oc+π/2   (Eq. 7)

Using this change of variables, the calculated load impedance at ω_o may then be expressed as:
Z_lc(ω_o)=|Z_lc(ω_o)|exp[j(π/2−φ)]=[ωL_mc+jR_e][ω_oL_mcR_e⁻¹+R_e(ω_oL_mc)⁻¹]⁻¹    (Eq. 8)

This expression for the calculated load impedance is illustrated in the dashed triangle, inspection of which shows that L_mc=R_eω_o⁻¹ tan φ. To derive an expression for the calculated gain, an expression is first derived for the calculated impedance ratio Z_rc(ω_o)/Z_lc(ω_o) in terms of φ, ω_o, and the calculated resonant angular frequency ω_rc:
Z_rc(ω_o)Z_lc(ω_o)⁻¹=−jωL_rc[(ω_rc/ω_o)²−1](R_e+jω_oL_mc)(jω_oL_mcR_e)⁻¹=−jL_rcL_mc⁻¹[(ω_rc/ω_o)²−1](1+j tan φ)=1−L_rcL_mc⁻¹[(ω_rc/ω_o)²−1](cos φ+j sin φ)(cos φ)⁻¹=−L_rcL_mc⁻¹[(ω_rc/ω_o)²−1](cos φ)⁻¹exp(jφ)   (Eq. 9)

Since the LLC converter is being designed to operate in boost mode in that the calculated angular resonant frequency ω_rc corresponds to the design parameter for the maximum switching frequency, the zero-angle operating frequency ω_o will be less than the calculated angular resonant frequency ω_rc and the term [(ω_rc/ω_o)²−1] will be greater than zero. Using exp(jπ)=−1 and taking −π/2≦φ≦π/2, Equation 9 may be rewritten as:
Z_rc(ω_o)/Z_lc(ω_o)=|L_rcL_mc⁻¹[(ω_rc/ω_o)²−1](cos φ)⁻¹|exp[j(π+φ)], −π/2≦φ≦π/2   (Eq. 10)

Thus, the angle of the calculated impedance ratio is π+φ when −π/2≦φ≦π/2. Because the gain is maximized at ω_o, therefore the calculated gain G_c(ω_o)=G_max[exp(jγ_oc)], where G_max is a scalar representing the maximum magnitude of the calculated gain. Since the angle of the calculated gain γ_oc=−φ+π/2, therefore the reciprocal of the calculated gain may be expressed as:
G_c(ω_o)⁻¹=G_max⁻¹exp[−j(−φ+π/2)]=1+|Z_rc(ω_o)/Z_lc(ω_o)|exp[j(π+φ)], −π/2≦φ≦π/2   (Eq. 11)

This expression for the gain in Equation 11 is illustrated in the dotted triangle of FIG. 3. Inspecting the dotted triangle provides the following two equations:
φ=a sin(G_max⁻¹)   (Eq. 12)
|Z_rc(ω_o)/Z_lc(ω_o)|=|L_rcL_mc⁻¹[(ω_rc/ω_o)²−1]|(cos φ)⁻¹=cos(φ)   (Eq. 13)

Therefore, the following equation for the calculated resonant inductance is also true:
L_rc=L_mc cos²(φ)[(ω_rc/ω_o)²−1]⁻¹   (Eq. 14)

As an alternative to this graphical demonstration, a complex-variable expression for the reciprocal maximum gain G_max⁻¹ can be derived directly:
G_max⁻¹=exp[j(π/2−φ)]G_c(ω_o)⁻¹=exp[j(π/2−φ)](1+Z_rc(ω_o)Z_lc(ω_o)⁻¹)=exp[j(π/2−φ)][1+exp[j(φ+π)]|L_rcL_mc⁻¹[(ω_rc/ω_o)²−1]|(cos φ)⁻¹]=[exp[j(π/2−φ)]+exp[j(3π/2)]|L_rcL_mc⁻¹[(ω_rc/ω_o)²−1]|(cos φ)⁻¹=sin(φ_c)+j[cos(φ)−|L_rcL_mc⁻¹[(ω_rc/ω_o)²−1]|(cos φ)⁻¹]]  (Eq. 15)

Since G_max⁻¹ is a scalar with no imaginary component, it follows that the imaginary component of the right-hand expression of Equation 15 must be equal to zero:
cos(φ)−L_rcL_mc⁻¹[(ω_rc/ω_o)²−1]cos φ⁻¹=0   (Eq. 16)

By noting that ω_o is chosen to be approximately equal to 2πf_min and the maximum switching frequency is chosen such that ω_rc≈2πf_max, the following expressions may be derived:
L_rc=L_mc cos²(φ)[(ω_rc/ω_o)²−1]⁻¹≈L_mc cos²(φ)[(f_max/f_min)²−1]⁻¹   (Eq. 17)
C_rc=L_rc⁻¹ω_rc⁻²≈L_rc⁻¹(2πf_max)⁻²   (Eq. 18)

Recalling Equations 7 and 8, the calculated magnetizing inductance may also be expressed as:
L_mc=R_eω_o⁻¹ tan φ≈R_e(2πf_min)⁻¹tan (φ)   (Eq. 19)

With the imaginary component of the right-hand expression of Equation 15 set to zero, it then follows that G_max⁻¹=sin(φ). Since G_max≈V_maxV_min⁻¹ and n_c≈V_max(sV_o)⁻¹, therefore:
φ=a sin(G_max⁻¹)≈a sin(V_minV_max⁻¹)≈a sin(V_min(n_csV_o)⁻¹)   (Eq. 20)

FIG. 4 shows an embodiment method for designing the power converter system of FIG. 1 in accordance with embodiments of the present invention. At step 402, at least one design system processor receives a list of available components and a set of power converter design parameters. These design parameters may be received as a first set of cells in a spreadsheet. The design parameters include the minimum DC input voltage V_min and the maximum DC input voltage V_max, as well as the minimum switching frequency f_min and the maximum switching frequency f_max of the switching bridge 102, a target output voltage V_o, a target output power P_o, and the switching factor s. At step 404, the design system then calculates a calculated power converter configuration, which includes the calculated magnetizing inductance L_mc, the calculated resonant inductance L_rc, the calculated resonant capacitance C_rc, and the calculated turns ratio n_c. Each of these calculated component values are calculated as previously described in reference to FIG. 1 above.

At step 406, the design system graphically displays the calculated power converter configuration at a user terminal. For example, the system may display a second set of cells in the spreadsheet, where the values of the second set of cells include the calculated power converter configuration. At 408, the design system writes the calculated power converter configuration to a data file, which may be a spreadsheet or any form of data file.

At step 410, the design system selects actual components based on the list of available components as well as the calculated power converter configuration, and applies these actual component values to a layout of a physical circuit for the power converter. The layout component values include an actual turns ratio n, an actual magnetizing inductance L_m, an actual resonant inductance L_r, and an actual resonant capacitance C_r that are each within plus or minus 25% of their respective calculated values n_c, L_mc, L_rc, and C_rc.

In a first embodiment, the layout components are selected from the list of available components such that the layout component values are each as close as possible to the calculated component values of the calculated power converter configuration. In a second embodiment, the layout components are selected from the list such that the layout component values jointly maximize a figure of merit when compared to the calculated values of the calculated power converter configuration. This figure of merit could be, for example, an absolute difference between f_max and a resonant frequency of the selected components, a maximum likelihood, an average percent error, or a weighted metric based on the dollar, space, or power requirements of components.

Referring again to FIG. 4, at step 414, the design system then synthesizes the physical power converter circuit based on the layout.

Referring now to FIG. 5, a block diagram of a processing system is shown that may be used for implementing some of the devices and methods disclosed herein. Specific devices may utilize all of the components shown, or only a subset of the components, and levels of integration may vary from device to device. Furthermore, a device may contain multiple instances of a component, such as multiple processing units, processors, memories, transmitters, receivers, etc. In an embodiment, the processing system includes a computer workstation. The processing system may include a processing unit equipped with one or more input/output devices, such as a speaker, microphone, mouse, touchscreen, keypad, keyboard, printer, display, and the like. The processing unit may include a CPU, memory, a mass storage device, a video adapter, and an I/O interface connected to a bus. In an embodiment, multiple processing units in a single processing system or in multiple processing systems may form a distributed processing pool or distributed editing pool.

The bus may be one or more of any type of several bus architectures including a memory bus or memory controller, a peripheral bus, video bus, or the like. The CPU may include any type of electronic data processor. The memory may include any type of system memory such as random access memory (RAM), static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), read-only memory (ROM), a combination thereof, or the like. In an embodiment, the memory may include ROM for use at boot-up, and DRAM for program and data storage for use while executing programs.

The mass storage device may include any type of storage device configured to store data, programs, and other information and to make the data, programs, and other information accessible via the bus. The mass storage device may include, for example, one or more of a solid state drive, hard disk drive, a magnetic disk drive, an optical disk drive, or the like.

The video adapter and the I/O interface provide interfaces to couple external input and output devices to the processing unit. As illustrated, examples of input and output devices include the display coupled to the video adapter and the mouse/keyboard/printer coupled to the I/O interface. Other devices may be coupled to the processing unit, and additional or fewer interface cards may be utilized. For example, a serial interface such as Universal Serial Bus (USB) (not shown) may be used to provide an interface for a printer.

The processing unit also includes one or more network interfaces, which may include wired links, such as an Ethernet cable or the like, and/or wireless links to access nodes or different networks. The network interface allows the processing unit to communicate with remote units via the networks. For example, the network interface may provide wireless communication via one or more transmitters/transmit antennas and one or more receivers/receive antennas. In an embodiment, the processing unit is coupled to a local-area network or a wide-area network for data processing and communications with remote devices, such as other processing units, the Internet, remote storage facilities, or the like. The network interface may be configured to have various connection-specific virtual or physical ports communicatively coupled to one or more of these remote devices.

Illustrative embodiments of the present invention have the advantage of providing techniques for designing LLC converters that operate within a specific range of switching frequencies in order to reduce interference from electromagnetic signals and to switching losses and component size. In some embodiments, the use of spreadsheet software tools allow LLC converter designers to rapidly calculate appropriate inductance, capacitance, and turns ratio values. Other embodiment systems may use, for example, EDA software tools that allow designers of electronic systems to design and analyze LLC power converters as an integral part of the design flow for an entire semiconductor chip.

While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.

Claims

1. A method for designing an inductance-inductance-capacitance (LLC) power converter, the method comprising:

receiving, by a processor, power converter design parameters of the LLC power converter, wherein the LLC power converter comprises a switching bridge coupled to a primary winding of a transformer, a resonant inductor and a resonant capacitor coupled in series between the switching bridge and the primary winding of the transformer, and a secondary side circuit coupled to a secondary winding of the transformer, the power converter design parameters comprising: a minimum DC input voltage Vmin and a maximum DC input voltage Vmax to be received by the switching bridge, a minimum switching frequency fmin and a maximum switching frequency fmax of the switching bridge, and a target output voltage Vo and a target output power Po to be output by the secondary side circuit;

calculating, by the processor, a first power converter configuration comprising: a calculated magnetizing inductance Lmc of the primary winding equal to Retan(φ)(2πfmin)−1, wherein φ is a load angle complement equal to asin(VminVmax−1), and Re is an equivalent reflected load resistance of the power converter, a calculated resonant inductance Lrc of the resonant inductor equal to Lmccos2(φ)(fmax2fmin −2−1)−1, and a calculated resonant capacitance Crc of the resonant capacitor equal to Lrc −1(2πfmax)−2; writing, by the processor, the first power converter configuration to a non-transitory computer readable medium; determining layout component values based on the first power configuration; and physically implementing the LLC power converter using the layout component values.

2. The method of claim 1, wherein

the power converter design parameters further comprise a switching factor s that is equal to 2 when the switching bridge has a half-bridge configuration and that is equal to 1 when the switching bridge has a full-bridge configuration; and

the first power converter configuration further comprises a calculated turns ratio nc of the primary winding relative to the secondary winding of the transformer comprised in the power converter, wherein nc is not less than 0.99Vmax(sVo)−1 and is not greater than Vmax(sVo)−1; and the equivalent reflected load resistance Re equals 8π−2nc2Vo2Po−1.

3. The method of claim 1, further comprising graphically displaying the first power converter configuration at a user terminal.

4. The method of claim 3, wherein

the receiving the power converter design parameters comprises: receiving a first set of cells in a spreadsheet, wherein values of the first set of cells comprise the power converter design parameters; and

the graphically displaying the first power converter configuration comprises: displaying a second set of cells in the spreadsheet, wherein values of the second set of cells comprise the first power converter configuration.

5. The method of claim 1, wherein the non-transitory computer readable medium is a file.

6. The method of claim 5, further comprising:

receiving, by the processor, a list of available components; and

determining the layout component values comprises selecting, by the processor, layout component values in accordance with the list of available components and the first power converter configuration, the layout component values comprising: an actual turns ratio na of the transformer greater than c1nc and less than c2nc, wherein c1 is not less than 0.75 and c2 is not greater than 1.25, an actual magnetizing inductance Lm_a of the primary winding of the transformer greater than c1Lm_c and less than c2Lm_c; an actual resonant inductance Lr_a of the resonant inductor greater than c1Lr_c and less than c2Lr_c; and an actual resonant capacitance Cr_a of the resonant capacitor greater than c1Cr_c and less than c2Cr_c.

7. The method of claim 6, wherein the selecting the layout component values comprises at least one of:

selecting layout components from the list of available components such that the layout component values are closest to component values in the first power converter configuration; and

selecting layout components from the list of available components such that the layout component values jointly maximize a figure of merit in accordance with the first power converter configuration.

8. A power converter design system comprising a non-transitory computer-readable medium storing programming, wherein the programming comprises instructions to:

receive power converter design parameters of an inductance-inductance-capacitance (LLC) power converter, wherein the LLC power converter comprises a switching bridge coupled to a primary winding of a transformer, a resonant inductor and a resonant capacitor coupled in series between the switching bridge and the primary winding of the transformer, and a secondary side circuit coupled to a secondary winding of the transformer, the power converter design parameters comprising: a minimum DC input voltage Vmin and a maximum DC input voltage Vmax to be received by the switching bridge, a minimum switching frequency fmin and a maximum switching frequency fmax of the switching bridge, and a target output voltage Vo and a target output power Po to be output by the secondary side circuit; calculate a first power converter configuration comprising: a calculated magnetizing inductance Lmcof the primary winding equal to Retan(φ)(2πfmin)−1, wherein φ is a load angle complement equal to asin(VminVmax−1), and Re is an equivalent reflected load resistance of the power converter, a calculated resonant inductance Lrc of the resonant inductor equal to Lmccos2(φ)(fmax2fmin−2−1)−1, and a calculated resonant capacitance Crc of the resonant capacitor equal to Lrc−1(2πfmax)−2;

write the first power converter configuration to a non-transitory computer readable medium;

determine layout component values based on the first power configuration; and physically implement the LLC power converter using the layout component values.

9. The system of claim 8, wherein

the power converter design parameters further comprise a switching factor s that is equal to 2 when the switching bridge has a half-bridge configuration and that is equal to 1 when the switching bridge has a full-bridge configuration; and

the first power converter configuration further comprises a calculated turns ratio nc of the primary winding relative to the secondary winding of a transformer comprised in the power converter, wherein nc is not less than 0.99Vmax(sVo)−1 and is not greater than Vmax(sVo)−1; and the equivalent reflected load resistance Re equals 8π−2nc2Vo2Po−1.

10. The system of claim 8, wherein the programming further comprises instructions to graphically display the first power converter configuration at a user terminal.

11. The system of claim 10, wherein

the instructions to receive the power converter design parameters comprise instructions to receive a first set of cells in a spreadsheet, wherein values of the first set of cells comprise the power converter design parameters; and

the instructions to graphically display the first power converter configuration comprise instructions to display a second set of cells in the spreadsheet, wherein values of the second set of cells comprise the first power converter configuration.

12. The system of claim 8, wherein the non-transitory computer readable medium is a file.

13. The system of claim 12, wherein the programming further comprises instructions to:

receive a list of available components; and

determine the layout component values by selecting the layout component values in accordance with the list of available components and the first power converter configuration, the layout component values comprising: an actual turns ratio na of the transformer greater than c1nc and less than c2nc, wherein c1 is not less than 0.75 and c2 is not greater than 1.25, an actual magnetizing inductance Lm of the primary winding of the transformer greater than c1Lmc and less than c2Lmc; an actual resonant inductance Lr of the resonant inductor greater than c1Lrc and less than c2Lrc; and an actual resonant capacitance Cr of the resonant capacitor greater than c1Ccr and less than c2Crc.

14. The system of claim 13, wherein the instructions to select the layout component values comprise at least one of:

instructions to select the layout component values from the list of available components such that the layout component values are closest to component values in the first power converter configuration; and

instructions to select the layout component values from the list of available components such that the layout component values jointly maximize a figure of merit in accordance with the first power converter configuration.

15. The system of claim 13, wherein the programming further comprises instructions to:

apply the layout component values to the file, wherein the file is a layout representing a physical circuit for the power converter.

16. The system of claim 15, wherein the programming further comprises instructions to synthesize the physical circuit in accordance with the layout.

17. A power conversion system comprising:

a switching bridge comprising a plurality of switches coupled to a DC power source having a minimum input voltage Vmin and a maximum input voltage Vmax, wherein the switching bridge is configured to switch at a frequency that is not less than a minimum frequency fmin and that is not greater than a maximum frequency fmax;

a primary side circuit coupled to the switching bridge, the primary side circuit comprising a primary winding of a transformer; and

a secondary winding magnetically coupled to the primary winding through a core of the transformer, and

an output terminal coupled to the secondary winding and configured to supply an output voltage that is not greater than a maximum output voltage Vo and an output power that is not greater than a maximum output power Po;

wherein the transformer has a magnetizing inductance Lm such that Lm is greater than c1Re(2πfmin)−1tan(φ) and less than c2Re(2πfmin)−1tan(φ), wherein c1 is not less than 0.75 and c2 is not greater than 1.25, φ is a load angle complement equal to asin(VminVmax−1), and Re is an equivalent reflected load resistance;

wherein the primary side circuit has a resonant inductance Lr such that Lr is greater than c1Lm(fmax2fmin−2−1)−1cos2(φ) and Lr is less than c2Lm(fmax2fmin−2−1)−1cos2(φ); and

wherein the primary side circuit has a resonant capacitance Cr in series with the resonant inductance such that Cr is greater than c1Lr−1(2πfmax)−2 and less than c2Lr−1(2πfmax)−2.

18. The system of claim 17, wherein

the switching bridge comprises two switches; and

the primary winding has a number of turns that is a multiple n times a number of turns of the secondary winding, wherein n is greater than c1Vmax(2Vo)−1 and less than c2Vmax(2Vo)−1; and

Re is not less than 8(πs)−2(0.99Vmax)2Po−1 and not greater than 8(πs)−2Vmax2Po−1, wherein s is a switching factor equal to 2.

19. The system of claim 17, wherein

the switching bridge comprises four switches;

the primary winding has a number of turns that is a multiple n times a number of turns of the secondary winding, wherein n is greater than c1VmaxVo−1 and less than c2VmaxVo−1; and

Re is not less than 8π−2(0.99Vmax)2Po−1 and not greater than 8π−2Vmax2Po−1.